Nutritional Requirements in Agriculture

Although small amounts of most elements can be found in plants, less than 20 of these elements are known to be essential. In addition to the elements C, H and O which together constitute 95-99% of a plant, there are six other constituent elements, namely N, K, P, Ca, Mg and S which must be supplied as macronutrients. Other elements, namely Si, Co, Fe, Mn, Zn, Cu, Mo, B and Cl, termed 

The N and K contents of green plants are about eight times those of P and Mg, which in turn are 10 -10 times those of the micronutrients (Table 12.2). Excessive quantities of most macro- and micronutrients have a toxic effect however, if introduced into the immediate vicinity of the plant. Elements such as As, Cd, Hg, Pb and U are regarded as toxic and enviromnentally hazardous even in the lowest concentrations. 

Abundant supplies of the macronutrients C, H and O are available from water and atmospheric carbon dioxide, while nearly all the other essential elements are supplied via the soil solution supplied by the root system. Commercial fertilisers are used primarily to supply the elements N, P and K. Soils are not usually deficient in Ca, Mg and S, but these elements are in any case present in many types of commercial fertiliser. Fertiliser compounds of leading importance are 

Before the introduction of phosphate fertilisers, there were few soils in the world which were capable of supplying sufficient phosphate for a sequence of good crop yields. There are, however, still extensive areas of P-deficient soils, particularly in tropical and sub-tropical areas. 

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